This invention relates generally to methods and apparatus for enhancing performance of drum type boilers while reducing operational disruptions.
Disruptions in service of drum type boilers can lead to loss of revenue as a result of reduced power generation from a power plant. Further, disruptions in service can lead to reduced life of power plant components.
Drum level trips are the most common cause of trips and disruptions in a power station, especially from the Heat Recovery Steam Generator/Balance of (power) Plant (HRSG/BOP) area. A trip of a plant stresses and reduces the life of power plant equipment and machinery.
Drum type boilers are the most commonly used boilers in power generation plants. There are two trips from a boiler drum level. The extremely high level trip is related to the safety of the steam turbine from the possibility of a water carry over. In the event of an extremely high level trip, the steam turbine is tripped offline if it is a multi-shaft configuration of combined cycle. If the steam turbine is a single-shaft configuration, the unit as a whole is tripped. A low level trip can also occur to protect the boiler from damage from high temperature from the heat source without water covering heat sensitive parts of the boiler. For a low level trip, the heat source (e.g., the gas turbine in a combined cycle power plant) is tripped.
Oscillations in feedwater flow result in variation in the steam production from a drum. In multi-drum or pressure boilers, systems are linked together so that disturbances and oscillations in one drum may cause disturbances in other drums. In the case of drums with a feedwater control upstream of an economizer, water steams in the economizer when flow is reduced. This steaming causes fouling of the interior of the tubes in the economizer. For drums in which the feedwater control valve is between the economizer and the drum, the economizer does not steam at low water flow, but the water in the economizer can be heated above the saturation temperature so that the water flashes in the feedwater control valve which erodes its seat.
In at least one known control system for drum type boilers, at low steam flows, control is initiated as a single element control mode, i.e., only the levels needed to control the Feedwater flow into the drum and to control the level are monitored. This control can be sluggish as the level change lags behind changes in steam flow. However, at low steam flows, the single element control mode is appropriate as flow conversions at the low end of the range are less accurate due to square root extraction from differential pressure transmitters used for measuring flows. Thus, small changes in the steam flows do not affect the drum level controls to a great extent at this range of the flow.
When the steam flow reaches a higher level (e.g., to the order of 20–30%) of the range of the flows, the control mode is switched to three-element control. In this control mode, the feedwater flow is controlled by controllers that monitor three signals, namely, drum level, feedwater flow, steam flow. Two controllers are used. A main controller controls flow, in that the main controller attempts to closely match the feedwater flow into the drum to the steam flow. The change or deviation in the drum level from the normal operating level provides a trim function to this control. This arrangement is called a cascaded control loop.
The tuning of the cascaded control loop in at least one known configuration is performed by making the feedwater PID (Proportional Integral Derivative) controller very fast with a high integral component. The level PID has mostly proportional content. Adding a high integral component to such a configuration can lead to a control that is highly responsive to flow variations. In addition, when the level deviation error remains high, the integral component increases with time to correct or trim the feedwater flow to correct the drum level. Increasing the integral component provides satisfactory level control for both a steady state condition in which there is low deviation from the normal water level and for a process upset condition with high deviation. However, the feedwater control tends to be oscillatory and can take a long time to settle down.